1. Field of the Invention
This invention relates to the recovery of pure platinum and palladium from an impure substantially gold-free precious and base metal bearing acidic chloride or mixed chloride/sulphate solution, such as might be found in the parting plant of a base metals refinery, and more particularly, a copper electrorefinery.
2. Prior Art
Until relatively recently, the platinum group metals (PGM) were recovered by classical precipitation procedures, which involved many repeated precipitation/redissolution stages in order to obtain metal of the desired purity. These processes are extremely tedious and time-consuming, with metal being tied-up in process often for many months. In the middle and late seventies, the major precious metal refiners introduced new solvent extraction processes in order to produce refined metal of higher purity, more quickly, and at lower cost. These processes have recently been reviewed in an article entitled Recovery and separation of platinum and palladium by co-extraction and differential stripping, in Separation Processes in Hydrometallurgy, (G. A. Davies, Editor), SCI, London 1987, p. 174. The processes used for palladium are based on either oxime (such as used by Johnson Matthey) or on alkyl sulphide (as used by INCO) extractants, both of which suffer from a number of drawbacks. The oxime extractants exhibit slow kinetics, are not highly selective, and require modifiers to make them effective. The alkyl sulphide extractants, whilst being very selective, have very slow kinetics, which mitigate against continuous operation. The loaded organics also have to be stripped with ammonia solutions, which dictates that it is essential to remove all traces of gold down to &lt;1 mg/L in the feed, since ammonia and either soluble or metallic gold can form potentially explosive solutions.
No completely satisfactory extractants have been reported for platinum. INCO uses TBP, which is acceptable only if there is a high level of platinum in the feed solution and if the base metal content is low.
Recently, Demopoulos and co-workers have patented the results of studies on the use of substituted 8-hydroxy quinoline derivatives for the recovery of both platinum and palladium (U.S. Pat. No. 4,654,145 which was issued Mar. 31, 1987). These reagents have the advantages that extraction and stripping kinetics are considerably faster than with oximes and alkyl sulphides, thereby permitting continuous operation, and that ammonia is not required for stripping.
The information published in the above patent indicates the potential of the substituted 8-hydroxy quinoline reagents. The reagents were tested for aqueous systems whereby gold, palladium and platinum were co-extracted, and then each metal selectively recovered from the loaded organic. Gold was recovered by third phase formation (i.e. by directly precipitating the metal from the organic with hot water). However, this is not a desirable operation for commercial practice, and since platinum stripping is also effected by water, there is a strong possibility of producing gold contaminated with platinum, and of losing platinum to the gold circuit. Hence, it is preferable to use feed solutions in which the gold has been preferentially removed, for example by sulphur dioxide precipitation.
Platinum and palladium can be co-extracted into the organic phase, although the published data show that high acidity is required for effective platinum extraction and low acidity for palladium extraction. Demopoulos recommends 2-3M hydrochloric acid. Whilst the reagent is claimed to be reasonably selective, it is clear from the published data that significant quantities of copper (in particular), silver, and other base metals typically found in process streams originating from copper electrorefinery feedstocks can be co-extracted. If not removed, these impurities eventually report to the refined metals. In our own testwork, it has also been noted that these reagents co-extract large quantities of acid, which has an appreciable effect on the platinum stripping circuit.
Selective stripping, or direct hydrogen reduction of the loaded organic has been described in the above patent for the recovery of platinum and palladium from the loaded organic. Hydrogen reduction is not an attractive proposition on a commercial scale because of the many operational problems it creates. The proposed method for stripping platinum is to use several stages of water. This is only a feasible proposition at low platinum loadings, since the strip liquor very quickly becomes acidic, and back extraction occurs, generally at pH&lt;1.5. Therefore, it is extremely difficult to generate a strip solution with any degree of platinum concentration.
Palladium stripping can be effected in several stages by 2-8M hydrochloric acid. In practice, it has been found that six stages are required with an acidity of 8M acid, otherwise the loaded organic is not fully stripped.